1. Field of the Invention
The present invention relates to a solar cell employed in a photovoltaic generator in which a plurality of the solar cells are interconnected in series, and a fabrication method thereof. More particularly, the present invention relates to a technique for preventing the solar cells from breaking down by a reverse voltage applied thereto when a part of the solar cells interconnected in series is shaded.
2. Description of the Prior Art
The fundamental structure of a conventional solar cell is a diode having a p-n junction. Accordingly, when it is employed in a practical power generator, a plurality of the solar cells should be interconnected in series so that the total voltage obtained by adding the voltages generated in respective solar cells reaches a desired voltage level.
When a part of the solar cells interconnected in series is shaded during power generating operation, only the shaded solar cell loses its power generating function. In this case, voltage generated by the other solar cells interconnected in series is applied to the shaded solar cell as a reverse voltage of the diode thereof. As a result, when the reverse breakdown voltage of the solar cell is low, the shaded solar cell is broken down, so that the function thereof required for photovoltaic power generation is lowered or lost. In order to prevent the shaded solar cell from breaking down, the reverse breakdown voltage of the solar cell should be increased, or alternately, another diode should be provided in anti-parallel to each group of solar cells whose generating voltage in total is lower than the reverse breakdown voltage of the individual solar cell.
Increase of the reverse breakdown voltage of the solar cell may be achieved by decreasing the impurity concentration in the base layer thereof. In general, a solar cell should have a p-n junction provided in a shallow region from the light receiving surface of the same, and especially in a solar cell used in cosmic space, the p-n junction must be provided in a shallow region separated from the light receiving surface by a distance of 0.3 .mu.m to 0.5 .mu.m or less. Although it is possible in a laboratory to form such a p-n junction by a diffusion process with respect to the base layer having low impurity concentration required for a reverse breakdown voltage of several hundred volts, this process is hardly applicable to mass production of solar cells. Especially in a GaAs solar cell, it is difficult to obtain low impurity concentration in the crystal growth process, and therefore, a reverse breakdown voltage over several ten volts cannot be obtained. Thus, increase of the reverse breakdown voltage of a solar cell is difficult by use of conventional techniques, and consequently, conventional solar cells are rarely employed in a high voltage generation system.
On the other hand, the insertion of the aforementioned additional diode is effective for the protection of the solar cell. However, this insertion brings an increase of the number of components, so that the fabrication cost of the solar cell is increased and the reliability on the system is lowered. Especially, the low reliability is a serious problem in cosmic space or like use of solar cells in which extremely high reliability is required.